Investigations on condensed metal vapour on the surface of alumina ceramics within vacuum interrupters after switching

Abstract

Basic investigations have been carried out on the dielectric performance of the ceramic (high-purity alumina, Al/sub 2/O/sub 3/) surface in vacuum interrupters after switching. In order to examine the influence of the shielding on the protection of the ceramic surface against metal vapour condensation different types of vacuum interrupters (VIs) have been tested: VIs with and without shielding. Additionally, two contact materials CuCr 25 wt.-% and WCAg 40 wt.-% have been investigated to compare the adhension of different metal vapours to alumina ceramic surfaces. After having performed a high-voltage conditioning of the VIs, DC arcs with arbitrary arcing times were triggered between the contacts simulating the generation of metal vapour during high current interruption and load break switching. Between the arcing tests the insulation levels of all VIs have been tested by means of an AC high voltage source. Afterwards the VIs were opened and the microstructure of the metallic condendate on the inner ceramic surface was analysed by means of scanning electron microscopy (SEM). The integral chemical composition of the metallic film was investigated by inductive coupled plasma (ICP). The conductivity of the surface condensate resulting from the condensation of the metal vapour was measured in order to compare the material investigations with the electrical behaviour of the surface.